Several acronyms are used throughout the text and are presented in TABLE 1 below for ease of reference.
TABLE 1ACRONYMDEFINITIONAMPSAdvanced Mobile Phone ServiceANTAntennaASICApplication-Specific Integrated CircuitBJTBipolar Junction TransistorDAMPSDigital Advanced Mobile Phone Service(see TDMA)EDGEEnhanced Data-rates for GSM/Global EvolutionFCCFederal Communications CommissionFETField-Effect TransistorGAITGSM ANSI-136 Interoperability TeamGPRSGeneral Packet Radio ServiceGSMGlobal System for Mobile communicationsHBHigh BandLBLow BandLNALow Noise AmplifierLOLocal OscillatorPHEMTPseudomorphic High Electron Mobility TransistorPINP-type-Insulator-N-typeRFRadio FrequencyRXReceiveSPDTSingle-Pole Double-ThrowTDMATime Division/Domain Multiple Access(see DAMPS)T/RTransmit/Receive SwitchTXTransmitVCOVoltage-Controlled Oscillator
GAIT (GSM ANSI-136 Interoperability Team) is a telecommunications industry working group whose mission is to develop standards to facilitate mobile terminal (i.e. cell phone) roaming among different mobile network technologies. GAIT mobile terminals are dual-band, tri-mode devices that allow users to roam on different network technologies using the same mobile terminal. GAIT mobile terminals can operate in AMPS, TDMA/DAMPS and GSM/GPRS modes in the 850 MHz band, and in TDMA/DAMPS or GSM/GPRS modes in the 1900 MHz band. DAMPS and TDMA refer to the same air interface standard.
The FCC promulgates standards with respect to spurious output signals emitted from mobile terminals. Spurious signal leakage is a form of radio frequency (RF) interference that occurs when signals other than the desired signal escape through a mobile terminal antenna port. The FCC standards ensure that unacceptable levels of RF interference are not emitted from a mobile terminal. As antenna interface switching within a mobile terminal becomes more complex (i.e. includes a greater number of ports), suppression of spurious output signals becomes increasingly difficult.
TDMA/DAMPS based prior art typically addressed spurious signal leakage problems by selectively placing a FET based switching system in a state that will suppress the spurious signal amplitude. Given a system with an RX to ANT isolation issue, the prior art solution would set the switch in a TX to ANT mode. This action isolates the RX and ANT ports but effectively closes the TX to ANT path. Simultaneous TX to ANT and RX to ANT isolation, however, was not achieved. In addition to isolation issues, linearity is also a concern. TDMA/DAMPS based prior art methods are unsatisfactory in a superheterodyne receiver-based GAIT mobile terminal, because the GSM TX mode has significantly higher harmonic suppression requirements than does TDMA/DAMPS, and thus requires superior switch linearity. Yet, present FET based switch systems do not have the required linearity margin.
What is needed is a system and method of suppressing spurious output signals without the usual system insertion loss or linearity issues.